chore(generation): add CTX_DIAG logs for silent-round investigation

Log num_ctx, message count, prompt/output tokens, headroom, and a
silent flag per round so we can correlate silent generations against
context pressure on the dev instance.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/fabledassistant/services/generation_task.py

@@ -412,8 +412,14 @@ async def run_generation(
 
             round_content_start = len(buf.content_so_far)
             round_tokens_start = timing.get("output_tokens") or 0
+            round_prompt_tokens_start = timing.get("prompt_tokens") or 0
+            approx_msg_chars = sum(len(str(m.get("content", ""))) for m in messages)
             effective_think = think
             retried_with_think = False
+            logger.info(
+                "CTX_DIAG round_start conv=%d round=%d num_ctx=%d msgs=%d approx_chars=%d think=%s",
+                conv_id, _round, num_ctx, len(messages), approx_msg_chars, effective_think,
+            )
 
             while True:
                 async for chunk in _stream_with_retry(messages, model, tools, effective_think, num_ctx=num_ctx):
@@ -519,6 +525,18 @@ async def run_generation(
 
                 round_content_added = len(buf.content_so_far) - round_content_start
                 round_tokens_added = (timing.get("output_tokens") or 0) - round_tokens_start
+                round_prompt_tokens = (timing.get("prompt_tokens") or 0) - round_prompt_tokens_start
+                headroom = num_ctx - round_prompt_tokens if round_prompt_tokens else None
+                is_silent = (
+                    not round_tool_calls
+                    and round_content_added == 0
+                    and round_tokens_added > 0
+                )
+                logger.info(
+                    "CTX_DIAG round_end conv=%d round=%d think=%s prompt_tokens=%d output_tokens=%d headroom=%s content_added=%d tool_calls=%d silent=%s",
+                    conv_id, _round, effective_think, round_prompt_tokens, round_tokens_added,
+                    headroom, round_content_added, len(round_tool_calls), is_silent,
+                )
                 if (
                     not round_tool_calls
                     and round_content_added == 0